1. Field of the Invention
The present invention relates to a discharge bulb including a cylindrical shroud glass welded integrally with, and surrounding an arc tube. More particularly, the invention relates to an arc tube body including a cylindrical shroud glass having 4000 to 7000 ppm of metal oxide added thereto.
2. Description of the Related Art
As shown in FIG. 7, a discharge bulb has such a structure that an arc tube 2 for filling a light emission substance such as Hg, NaI or ScI together with an Xe gas in a hermetic glass bulb 2a having electrodes 3 and 3 provided opposite to each other comprises an arc tube body 1 surrounded by a cylindrical shroud glass 4. The reference numeral 5 denotes a molybdenum foil sealed and attached to a pinch seal portion 2b of the arc tube and the reference numeral 6 denotes a lead wire to be led from the pinch seal portion 2b. 
A method of lighting the arc tube 2 of the discharge bulb includes a positive electrode lighting method for applying a positive voltage to the electrodes 3 and 3 of the arc tube to be discharged and a negative electrode lighting method for applying a negative voltage to the electrodes 3 and 3 of the arc tube to be discharged. As shown in FIGS. 8 and 9, the negative electrode lighting method is excellent in a chromaticity characteristic and has a long lifetime. As such, taking into account only the benefits of the lighting method from a bulb perspective, the negative electrode lighting method is better than the positive electrode lighting method. More specifically, FIG. 8 shows a chromaticity (x, y) characteristic in which the state of a change in the luminescent color of the arc tube is plotted every 500 hours. Referring to the chromaticity (x, y) characteristic, as shown in an arrow of FIG. 8, a change in the chromaticity (x, y) is higher (smaller), that is, the lifetime is shorter (longer) in the positive (negative) electrode lighting method with the passage of time. In respect of a ballast circuit which is indispensable to the light-up of the arc tube, however, the negative electrode lighting method has such a drawback that an inverting circuit for inverting a positive voltage once generated into a negative voltage is required, and therefore, the ballast circuit becomes complicated and large-sized which increases costs as shown in FIG. 9. The positive or negative electrode lighting method is employed depending on whether the lifetime of the bulb or an advantage on the ballast circuit side has a priority. The structure of the discharge bulb is entirely identical and any method can be employed.
In the development of the discharge bulb and ballast circuit of this kind, there is a problem in that the luminescent color of the arc tube becomes pale with time and a luminous flux is thereby reduced in the process of study in which the discharge bulb is to be further improved.
This cause was investigated. As a result, it was found that NaI and ScI in (a hermetic glass bulb of) the arc tube are ionized to be an Na+ ion and an Sc+ ion by the light-up of the arc tube (a discharge between the electrodes 3 and 3) as shown in FIG. 7 and the Na+ ion is smaller than the Sc+ ion and the molecule of a quartz glass (the molecule of the quartz glass constituting the arc tube and a shroud glass) and thereby passes through the side wall of the hermetic glass bulb 2a and that of the shroud glass 4 so that a red luminescent component (Na) in (the hermetic glass bulb of) the arc tube is decreased.
More specifically, the discharge bulb is inserted and attached to a reflector and is thus used. In some cases, a ground potential (0 volt) is present in the vicinity of the arc tube body 1, for example, a metallic shielding shade 8 for controlling light distribution is provided in the vicinity of the arc tube 2 or means for holding the inside of the reflector to the ground potential (0 volt) is provided in order to shield an electromagnetic wave generated during the light-up of the arc tube (the discharge between the electrodes) (an electromagnetic wave which is the cause of an electromagnetic noise in an electronic component, such as a car radio). In the positive electrode lighting method for applying a positive voltage to the electrode 3, particularly, the Na+ ion in the hermetic glass bulb 2a is pulled toward the ground potential (0 volt) side upon receipt of the influence of an electric field generated between the electrode 3 and the ground potential (the shielding shade 8) and thereby passes through the hermetic glass bulb 2a (and the shroud glass 4) as shown in an arrow of FIG. 7. Thus, the Na+ ion passes toward the outside of the arc tube.
To the shroud glass 4 is added metal oxide for absorbing (shielding) ultraviolet rays (hereinafter referred to as UV) within a wavelength range which is generally hazardous to a human body or metal oxide for preventing a devitrification phenomenon in which a halogen ion ionized in (the hermetic glass bulb of) the arc tube or a tungsten ion evaporated from an electrode reacts with a quartz glass (SiO2) and sticks as a white crystal to the inside of the hermetic glass bulb. The inventor decided to investigate whether or not an electric field acting in (the hermetic glass bulb 2a of) the arc tube can be shielded by the metal oxide added to the shroud glass. As a result of experimentation, it was confirmed that the addition of metal oxide in a predetermined amount is effective for suppressing the passage of the Na+ ion toward the outside of the arc tube, therefore, resulting in the present invention.
The invention has been made in consideration of the problems of the related art based on the knowledge of the inventor and has an object to provide a discharge bulb in which a predetermined amount of metal oxide is added to a shroud glass surrounding an arc tube, thereby reducing the influence of an external electric field acting on (the hermetic glass bulb of) the arc tube by an electrostatic shielding function and suppressing the passage of an Na+ ion toward the outside of the arc tube.
In order to achieve the object, a first aspect of the invention is directed to a discharge bulb comprising an arc tube body having such a structure that a cylindrical shroud glass having metal oxide added thereto is welded integrally to surround an arc tube sealing Hg, NaI, ScI and Xe gases, wherein a total amount of addition of the metal oxide in the shroud glass ranges from 4000 to 7000 ppm.
A predetermined amount of metal oxide added to the shroud glass has such a configuration as to cover (the hermetic glass bulb of) the arc tube, thereby shielding (the hermetic glass bulb of) the arc tube against an external electric field (which will be hereinafter referred to as an electrostatic shielding function) and lessening the influence of the external electric field to act on (the hermetic glass bulb of) the arc tube. For this reason, the influence of the external electric field caused by the presence of a ground potential (0 volt) in the vicinity of the arc tube which acts on an Na+ ion in (the hermetic glass bulb of) the arc tube (force for pulling the Na+ ion in the direction of the presence of the ground potential (0 volt)) is reduced so that the Na+ ion cannot pass through (the hermetic glass bulb of) the arc tube and the shroud glass.
The metal oxide contained in the shroud glass does not have a sufficient electrostatic shielding function for lessening the influence of the external electric field for (the hermetic glass bulb of) the arc tube if a total amount of addition is less than 4000 ppm. On the other hand, when the amount of the addition exceeds 7000 ppm, stripe-shaped concavo-convex portions are generated on the surface of the shroud glass or the conformability to the arc tube is deteriorated. Thus, the molding property and adhesion of the shroud glass is reduced. Consequently, it is desirable that the amount of metal oxide should range from about 4000 to 7000 ppm.
A second aspect of the invention is directed to the discharge bulb according to the first aspect of the invention, wherein the amount of addition of the metal oxide contained in the shroud glass has 1500 ppm or more of Al2O3 and 2500 ppm or more of CeO2.
In general, a proper amount of Al2O3 is added to the shroud glass in order to prevent a devitrification phenomenon in which a halogen ion ionized in (the hermetic glass bulb of) the arc tube and an evaporated tungsten ion react with a quartz glass (SiO2) to stick as a white crystal to the inside of the hermetic glass bulb. If the amount of Al2O3 is less than 1500 ppm, the devitrification phenomenon can be prevented effectively and the electrostatic shielding function cannot be obtained sufficiently. Consequently, the passage of Na towards the outside of the arc tube can not be prevented effectively. Moreover, the shroud glass has an insufficient chemical durability and mechanical hardness.
Moreover, if CeO2 is less than 2500 ppm, it is impossible to sufficiently cut ultraviolet rays which are hazardous to a human body.
In order to effectively prevent the devitrification phenomenon, to sufficiently fulfill the electrostatic shielding function, to satisfy the chemical durability and the mechanical hardness, and to sufficiently cut the ultraviolet rays, accordingly, the amount of addition of Al2O3 is set to be 1500 ppm or more and the amount of addition of CeO2 is set to be 2500 ppm or more.
A third aspect of the invention is directed to the discharge bulb according to the first or second aspect of the invention, wherein a sealed space between the arc tube and the shroud glass is filled with an inert gas (for example, Ar or Kr) having a pressure of one atmosphere or higher.
The inert gas (Ar or Kr) having a greater molecular weight than the molecular weight of air (≈ the molecular weight of N2) is present at one atmospheric pressure or more in a sealed space around (the hermetic glass bulb of) the arc tube. Consequently, a dielectric constant in the sealed space around the arc tube is increased and the external electric field acting on the arc is reduced so that the Na+ ion correspondingly passes with difficulty.
A fourth aspect of the invention is directed to the discharge bulb according to any of the first to third aspects of the invention, wherein a metallic shielding shade for controlling light distribution is provided in the vicinity of the arc tube body and the shielding shade has an electromagnetic wave shielding function.
The metallic shielding shade for controlling light distribution which is held to the ground potential (0 volt) shields an electromagnetic wave generated from the arc tube, thereby inhibiting the generation of an electromagnetic noise in an electronic component. The presence of the metallic shielding shade (ground potential) provided in the vicinity of the arc tube body causes an electric field to act on an arc, thereby pulling the Na+ ion toward the shielding shade side. By the electrostatic shielding function of the metal oxide added to the shroud glass, the electric field acting on the arc is small so that the passage of the Na+ ion toward the outside of the arc tube can be suppressed.